1. Field of the Invention
This invention relates to flowmeters and, more particularly, to a flowmeter for accurate measurement of fluid flow rates which vary significantly over short periods of time. The invention is useful for monitoring changes in fluid flow through pipes generally.
2. Description of the Prior Art
In the past, various instruments have been designed to accurately measure fluid flow within limited ranges of flow rate. These devices include wet test meters, dry test meters, volumetric gas meters, hot wire flowmeters and rotameters. Such devices in combination may be used to monitor flow rates over many desirable ranges of measurement. However, a limiting feature of such prior designs sometimes arises when it is necessary to measure fluctuating flow rates which extend outside the dynamic range of a single instrument during brief time intervals. Circumstances which demonstrate the need for overcoming this limitation are illustrated by the prior efforts made to closely monitor exhaust air flows in steam turbine generating systems. Such systems are characterized by exhaust flow rates varying from 0 to 400 cubic feet per minute (CFM). The desirable range of flow rates in certain steam turbines during normal operating conditions is between one and ten CFM. The exhaust pipes in such systems are typically six inches in diameter and may be larger for long lengths of pipe in order to minimize back pressure which would otherwise occur during startup when the turbine system contains large quantities of air. This air discharge can result in initial exhaust rates on the order of 400 CFM. In the past flowmeters which have been used for measuring these exhaust rates have required a minimum fluid flow velocity through the measurement pipe of approximately 50 feet per minute in order to maintain an acceptable level of accuracy. However, the velocity of a 1 CFM flow through a six inch pipe is on the order of only five feet per minute. Consequently, one problem has been that of accurately measuring the exhaust flow in the desirable range during normal operating conditions without creating excessive back pressures during startup and other periods during which the exhaust air flow rate increases considerably beyond the desirable range of one to ten CFM.
A prior method which has been used to measure the exhaust air flow rate from a steam turbine is illustrated in FIG. 1. A valve V closes the exhaust pipe P and bypasses the exhaust air through a flowmeter F. The measurement tube T of the flowmeter has a markedly smaller inside diameter than the exhaust pipe, thus increasing the exhaust air flow velocity above the minimally acceptable velocity needed to measure the volumetric flow of the gas within an acceptable level of accuracy. A first feature of this bypass method which is believed to be disadvantageous is that the valve must completely seal the pipe in order to sustain accurate flowmeter measurements. Failure of the valve to create an adequate seal will result in spurious data. A second disadvantageous feature of this bypass method is that when the exhaust flow rate increases, e.g., due to a sudden leakage of air into the turbine system, the pipe valve must be opened in order to minimize back pressure. Thus, the bypass method is not useful for continuously monitoring the leakage of air into the turbine system by measuring the flow rate of air being pumped out of the system. However, it is desirable to continuously monitor such air leakage so that developing problems can be discerned early and corrected before significant damage is done to the system. Alternative solutions such as monitoring pressure fluctuations in the exhaust pipe require measurement sensitivity on the order of 3.times. 10.sup.-6 p.s.i. under ambient pressure conditions of fifteen to twenty p.s.i. Reliable instrumentation of suitable sensitivity has not heretofore been available to monitor the flow rates of gases over the entire range which is typical of steam turbine exhaust air pipes. Generally, it is believed that there is a need for a device which continuously monitors the flow rates of fluids which are characterized at times by relatively small flow velocities as a result of a relatively large pipe diameter, but which flow velocities may at times increase beyond 50 feet per minute.